Electrophotographic copiers capable of variable magnification are well known in the art. Generally, the optical system of such a copier includes a lens for forming a focused image of at least a strip portion of an original document on the photoconductor. An electrostatic latent image corresponding to that of the original is formed on the photoconductor, which has been previously uniformly electrically charged, by moving the photoconductor at a uniform velocity through an exposure station while simultaneously effecting relative movement between the optical system and the original document. This relative movement may be accomplished either by moving the document past an optical system consisting of fixed elements or by using one or more movable mirrors to scan a stationary original. To change the reproduction ratio of such an optical system one must alter both the object distance between the lens and the original document and the image distance between the lens and the photoconductor. Generally, this is accomplished by moving either the lens or an image-side mirror to alter the image distance while concomitantly moving an object-side mirror to alter the object distance.
One of the problems inherent in variable-magnification copiers of the prior art is that of maintaining the exposure of the photoconductor surface constant for various selected magnifications. In general, for a constant document illumination the brightness of the optical image on the photoconductor varies inversely with the square of the image distance between the lens and the photoconductor. This distance in turn varies with the selected magnification. Previous systems have compensated for this variation in brightness of the optical image by varying the width of a transversely extending optical slit, adjacent to the photoconductor, in accordance with the selected magnification. Ikeda et al U.S. Pat. No. 4,125,323 discloses one such system in which an eccentric cam coupled to a lens-positioning motor engages a follower coupled to a shutter member adjacent to the photoconductor. While this system does provide some exposure correction for variations in brightness due to changes in image distance, the particular system disclosed is relatively complicated mechanically. In addition, the contour of the cam is such that the actual compensation can equal the required compensation at only a limited number of magnifications. While this may be adequate in a system such as the one disclosed, in which only two reproduction ratios are contemplated, it would fail to provide accurate exposure correction in a system in which the lens position is continuously adjustable to provide continuously variable magnification. Any discrepancy between the actual exposure correction and the desired correction would be particularly evident in a system in which the selected magnification varies between widely spaced limits, such as between 0.5 and 1.5.